In many industrial and chemical operations there are gas streams which contain an unwanted amount of acid gases, more particularly CO2, the amount of which must be reduced for further processing, for transportation or for the prevention of CO2 emissions.
On the industrial scale, CO2 is typically absorbed from a gas mixture by using aqueous solutions of alkanolamines as an absorption medium. The loaded absorption medium is regenerated by heating, depressurization to a lower pressure or stripping, and the carbon dioxide is desorbed. After the regeneration process, the absorption medium can be used again. These methods are described for example in Rolker, J.; Arlt, W.; “Abtrennung von Kohlendioxid aus Rauchgasen mittels Absorption” [Removal of carbon dioxide from flue gases by absorption] in Chemie Ingenieur Technik 2006, 78, pages 416 to 424, and also in Kohl, A. L.; Nielsen, R. B., “Gas Purification”, 5th edition, Gulf Publishing, Houston 1997.
A disadvantage of these methods, however, is that the removal of CO2 by absorption and subsequent desorption requires a relatively large amount of energy and that, on desorption, only a part of the absorbed CO2 is desorbed again, with the consequence that, in a cycle of absorption and desorption, the capacity of the absorption medium is not sufficient. In addition, during a desorption by heating, a thermal and oxidative breakdown of the amine occurs on the hot heat-exchange surfaces.
WO 2008/015217 proposes to use an absorption medium, showing a phase separation into two liquid phases upon heating, for a method for separating CO2 from gas mixtures in order to decrease the energy requirement for the desorption of CO2. In this method the CO2 is desorbed at a high CO2 partial pressure and so only an insufficient capacity of the absorption medium is achieved in a cycle of absorption and desorption.
US 2009/199709 and US 2010/104490 describe methods using such an absorption medium, where the absorption medium loaded with an acid gas is heated to form two liquid phases, these phases are separated and only the acid-gas-rich liquid phase is fed to a desorption column, while the liquid phase that is low in acid gas is returned directly to the absorption. In these methods, however, some of the gas that is bound in the loaded absorption medium is already liberated in the apparatus in which the phase separation proceeds. In practice, this leads to problems, since the acid gas is generally liberated from the heavier phase and ascending gas bubbles counteract the phase separation. In addition, the method of US 2009/199709 and US 2010/104490 may not be operated stably with the devices used in US 2009/199709 and US 2010/104490, if two liquid phases are also formed in the desorption column.
U.S. Pat. No. 4,251,494 describes a method using an absorption medium which comprises water, a sterically hindered amine and an alkali metal carbonate. In this method, the composition of the absorption medium is selected in such a manner, that after the desorption the absorption medium forms two liquid phases in the evaporator of the desorption column due to evaporation of water and the temperature rise, which are separated in the evaporator and are returned to the absorber at different points. The method requires an absorption medium having a high content of alkali metal carbonate, which has an undesirably high corrosivity. In addition, just as with the use of a single-phase absorption medium, a thermal and oxidative breakdown of the amine occurs on the hot heat-exchange surfaces of the evaporator.
There is therefore still a need for a method and a device for separating acid gases from a gas mixture, in which the energy requirement is reduced by forming two liquid phases during the desorption, and which does not have the disadvantages of the methods and devices known from the prior art.